Thyristors are non-linear solid state devices that are bistable; that is, they have both a high and a low impedance state. They are commonly four layer PNPN structures. Thyristors are usually switched from one impedance state to the other by means of a control or gating signal applied to one of the base regions.
The gate sensitivity of a semiconductor device is by definition inversely dependent on the gate current needed to fire the device. Gate current is in turn a function of the injection efficiency (.gamma.) into the cathode-base region and the carrier lifetime (.tau.) in said base. region of the device. Both of these parameters are affected by the impurity concentration (N.sub.A) in the base region. Thus, the gate current can be decreased and gate sensitivity increased by decreasing the cathode-base impurity concentration. Conversely, increasing the base impurity concentration to decrease gate sensitivity increases the forward voltage drop. Design of a gated semiconductor device has therefore routinely involved a trade-off between gate current and forward voltage drop requirements.
To provide this trade-off, a gallium diffusion has typically been made prior to diffusion of the cathode-emitter region. The purpose of this second cathode-base diffusion (the first usually being aluminum) is to lessen the injection efficiency of the emitter-base junction by raising the impurity concentration at the working point (see FIG. 3A). This practice prevents accidental firing of the device by stray currents in the gate circuit, the gate current to fire (I.sub.g) usually being in excess of 10 milliamps. Referring to FIG. 3A, the diffusion is typically symmetric for convenience. The presence of the gallium diffusion on the anode-emitter region does not contribute significantly to the performance of the device since the regrown region, formed in making the ohmic contact to the anode-emitter region, is usually penetrated to where the aluminum is the dominant impurity.
To control gate sensitivity, it has also been known to selectively diffuse gold into the gating portion of the cathode-base, or selectively irradiating the gating portion of the cathode-base of the thyristor, see our application Ser. No. 283,685, filed Aug. 25, 1972 and now U.S. Pat. No. 3,840,887. Both of these techniques result in reducing the minority carrier lifetime in the base region. The gate current to fire (I.sub.g) is thereby increased without increasing the forward voltage drop. Neither of these techniques resulted in a reduction of the forward voltage drop. To the contrary, irradiation of the conducting portion of a thyristor has been known to increase the forward voltage drop of the device, see our application Ser. No. 283,685, filed Aug. 25, 1972 and now U.S. Pat. No. 3,840,887. Also gold diffusion is known to increase the leakage current of the device.
The present invention overcomes these difficulties and disadvantages of the prior art. It provides a thyristor with a forward voltage drop lower than any heretofore attainable without the sacrifice of other critical parameters and particularly gate current to trigger (I.sub.g).